1) Is it possible to remove digital signatures from form?
ANSWER: A signature can only be removed ("unsigned") if the system or user has access to the "private" key used to generate the signature in the first place. For example, let's say User A signs a PDF... Only User A can unsign that PDF. If you were to use LC Digital Signatures ES to "unsign" a PDF, you would need to have all of the potential user "Credentials" and Credential passwords stored in the Trust Store so LC would have access to the private keys to be able to unsign a signature field. This is not very feasable if the number of potential signers is large.
2) Is there a simpler way to combine digital signatures and rights management
ANSWER: Combining Digital Signature and Rights Management is not complicated. You just need to be aware of the "Order of Operations" required. Always "Encrypt" first (Rights Mgt, Certificates, and Password can be used for encryption) then "Certify" (assuming you are Certifying the PDF), then add Reader Extension rights (assuming you want to extend functionality of the document for Reader)
The reason the above order is required... When you sign a document, a hash is generated based on the document, if you then encrypt that signed document, you are modifying the document which in turn causes a different hash to be generated... this breaks the signature.
As for the "Filling in form fields and signing" option in a policy, this is a "permission" that you can allow or disallow for PDF forms with a policy applied by RM. For example, A PDF has a policy applied where User A has the "Filling in form fields and signing" permission enabled andf User B does not. User A can open the form and interact with it by filling it in and or sign the form. User B would only be able to "view" the form. This permission is only relevant what using RM to protect fillable PDF forms. Also, it shouldn't be confused with the Reader Extensions permission of allowing Digital Signatures in Reader.
For example, If you wanted a "Certified" form to be filled in and signed by User A with Adobe Reader, you would need to:
Apply a policy to the PDF where User A had the "Filling in form fields and signing" permission enabled, then apply a "Certify" signature which had the "Allow Form Fill and Signing" permission enabled, then Reader Extend the PDF form that enables the "Digital Signatures" permission which activates the Digfital Signatures functionality in Reader for that particular form.
It may sound complicated, but it really isn't 
Regards
Steve